The present invention relates to a probe for determining an oxygen concentration in a gas mixture, in particular in the exhaust gas of internal combustion engines having the features set forth in the preamble of claim 1.
Previously proposed probes determine the oxygen concentration in the exhaust gas of internal combustion engines and are used to influence the setting of the fuel air mixture during operation of the engine. The fuel/air mixture may be in the rich range, i.e., there is an excess of fuel in stoichiometric terms, so that only a small quantity of oxygen relative to other partly unburned components is present in the exhaust gas. In the lean range, in which there is a greater quantity of oxygen relative to the air in the fuel/air mixture, the oxygen concentration in the exhaust gas is correspondingly high. If the fuel/air mixture is of stoichiometric composition, both the amount of fuel and the amount of oxygen in the exhaust gas are reduced.
Lambda sensors which detect a lambda value greater than 1 in the lean range, a lambda value less than 1 in the rich range, and a lambda value=1 in the stoichiometric range and which are used to determine the oxygen concentration in exhaust gas are known. In this case, the lambda sensor supplies a detection voltage in a known manner, which is conveyed to a circuit arrangement. In known probes, with the help of the circuit arrangement the detection voltage is converted into a pump voltage for a pump cell, which is also a component of the probe and is exposed to the exhaust gas. The pump cell, in which oxygen ions are pumped from an inner pump electrode to an outer pump electrode or vice versa based on the oxygen concentration present. Depending on whether the lambda sensor detects a rich range, i.e., a lambda value less than 1, or a lean range, i.e., a lambda value greater than 1, the circuit arrangement determines whether the outer pump electrode, which is connected to an active input of the circuit arrangement, is connected as a cathode or as an anode. The inner pump electrode of the pump cell is connected to ground, so that at the pump cell an anodic limit current flows in the case of rich measured gas or a cathodic limit current flows in the case of a lean measured gas. In the case of stoichiometric operation, i.e., if the lambda value=1, the pump voltage is close to 0, so that no limit current flows.
The detection voltage of the probe is determined via a Nernst measuring cell, which determines the difference between the oxygen concentration at a Nernst electrode and that at a reference electrode. The reference electrode is connected to a constant current source, while the Nernst electrode is connected to ground. As a result, the detection voltage is based correspondingly on the difference between the respective oxygen concentrations.
Because the Nernst electrode and the inner pump electrode of the probe are connected to ground, it is known that they can be connected to the circuit arrangement via a joint supply conductor. In this case, the electrodes are initially contacted inside the probe to separate printed conductors, which then come together inside the probe at a contact point to form the joint supply conductor.
By detecting the pump current of the pump cell required to maintain xcex=1 in a measuring space (hollow space) of the probe, it is possible to determine whether the fuel/air mixture used to operate the internal combustion engine is a rich or a lean mixture. If there is a change-over from a rich range to a lean range or vice versa, the pump current drops or increases, respectively. If the engine is being operated in the stoichiometric range, i.e., with a lambda value=1, the pump current has a jump point that marks the transition from the lean range to the rich range and vice versa, respectively.
Referring to FIG. 4, there is seen a conventional connectivity between a gas probe and an operational amplifier. In known probes, it is disadvantageous that because the supply conductor of the Nernst electrode and the inner pump electrode is shared, at least in some sections, their joint supply conductor resistor, which is not only part of the Nernst voltage circuit of the Nernst measuring cell but also part of the pump voltage circuit of the pump cell, causes coupling, which has an impact on lambda=1 ripple. This minimizes the counterswings and overswings in voltage that may occur in the event of a jump response in response to a transition from the rich range to the lean range.
By contrast, the probe according to the present invention has the advantage that negative feedback of the pump voltage circuit and the Nernst voltage circuit is optimized. Because a joint supply conductor resistor of the Nernst electrode and of the inner pump electrode is formed by a loaded voltage divider whose individual resistors are arranged so that negative feedback of a Nernst voltage circuit and of a pump voltage circuit is increased, the lambda=1 ripple can be reduced. The individual resistors are arranged so that when the detection voltage of the Nernst measuring cell transitions from the lean range to the rich range or vice versa, this produces a result via the jump point that triggers an anodic or cathodic limit current, respectively, via the pump cell, so that negative feedback via the joint supply conductor section of the Nernst measuring cell and the pump cell can be achieved.
According to a preferred embodiment of the present invention, an additional external resistor is connected in series to the joint supply conductor section of the Nernst measuring cell and the pump cell. Thanks to this additional external resistor, the total resistance of the joint supply conductor section is increased, so that at the constant current at which the Nernst measuring cell is operated the detection voltage is greater, so that the influence of negative feedback is increased by the cathodic or alternatively anodic limit current, which also flows through the additional resistor.
According to a further preferred embodiment of the present invention, a cross section of the joint supply conductor section is reduced. Reducing the cross section is another way to increase the resistance value of the joint supply conductor section, so that this is also a straightforward way of increasing negative feedback between the Nernst voltage circuit and the pump voltage circuit.
According to a further preferred embodiment of the present invention, the contact point where the printed conductor of the inner pump electrode meets the printed conductor of the Nernst electrode is moved spatially as close as possible to the electrodes, so that the length of the joint supply conductor section increases, so that the resistance of this joint supply conductor section is also increased by a defined amount.